This invention relates to a method for lidding open-top containers with elastomeric film without the application of heat.
The physical and mechanical properties of plastics make them particularly well suited as closuring or lidding material for the cup-shaped containers in general use particularly throughout the fast food industry. Plastic lids are impermeable to liquids, easily emplaceable, disposable, and relatively inexpensive. Many plastics of the thermoplastic, elastomer, and thermosetting groups have these desirable properties.
In the fast food industry, however, plastic lidding materials have been selected, almost exclusively, from among the thermoplastics. These thermoplastic materials have been used in several varieties of preformed lids and in a newer variety as a heat-shrunk film lid.
Speed of emplacement is an important factor in the choice of lid type. Preformed lids can be emplaced more quickly than the heat-shrunk films. A preformed lid has an outer skirt which, in a single step, is snapped over the top rim of the container. The thermoplastic film lid requires two steps: covering the container with an oversize piece of thermoplastic film and heat-shrinking the film against the periphery of the cup.
The most prominent shortcoming of the preformed lids is the inadequate seal between the lid and the container. This inadequacy manifests itself in several ways: first, the seal does not withstand jostling between the lid and other items; second, the juncture between the lid and the container is not leak-proof. Despite the poor seal, removing the stiff lids without spilling some of the contents can be troublesome. These lids have some additional drawbacks. The lidding process requires two hands and each cup size requires a specific lid size.
Some of the shortcomings of preformed lids are avoided by the use of film which is thermoformed or heat-shrunk over the top of the container. Such thermoplastic film may be emplaced with one hand by use of a counter-top lidding device designed for that purpose. In the current form, such counter-top devices first feed a fixed amount of a suitable thermoplastic film over the top of the container. The film, such as polyethylene, polyvinyl chloride, or polypropylene is then held over the top of the container while jets of hot air are blown against the edges or heat is applied by some other method. Because the heat edges of the film are in a somewhat fluid state, they collapse downwardly and are shrunk against the periphery of the cup.
Not all of the deficiencies of the preformed lids are obviated by such film lids; some, in fact, are increased. The heat necessary to effectuate the desired shrinkage can cause brittleness which weakens the film. A good heat-seal often makes the lid difficult to open. Contamination of the container in the heat-seal area with contents of the container can cause a poor seal. The heat-shrinking method also tends to be relatively slow. The demands of the industry for a fast and convenient method and appartus for emplacing strong, leakproof, easily removable lids have not been met with the heat-shrunk thermoplastics.